1. Field of the Invention
The present invention relates to a microscopic illumination apparatus which is applied to a microscope of a transmission type for bright field observation, and a microscopic apparatus with the microscopic illumination apparatus mounted thereon.
2. Related Background Art
On a microscope of the transmission type for bright field observation, there is mounted a microscopic illumination apparatus of a type of illuminating a sample from a position opposite to an objective lens with the sample being placed therebetween.
The primary performance required for such a microscopic illumination apparatus is a less unevenness in illumination (the uniformity of illumination) so that a Koehler illumination is employed.
A camera is often mounted on a microscope for recording a microscopic image. Recently, digital cameras are used because of an advanced performance of image pick-up elements such as a CCD. However, in an image obtained by a digital camera, even a slight difference in brightness is visualized so that a slight illumination unevenness in a Koehler illumination attracts attention.
For this reason, it is known that, in order to further improve the unevenness in illumination, an integrator such as a fly-eye lens or a rod that is employed in an illumination unit of a projection exposure apparatus is used, for a Koehler illumination.
FIG. 9 is a configuration example of a microscopic illumination apparatus 100 in which a fly-eye lens (having a fly-eye structure both on the light source side and the condenser lens side) is applied to a Koehler illumination.
Note that, in FIG. 9, a reference symbol O denotes a sample surface of the microscope, and a reference numeral 105 a condenser lens attached to the microscope.
First, the microscopic illumination apparatus 100 is, as being of the Koehler illumination, provided with a light source 101 and a collector lens 102 for collimating the light flux emitted from the light source 101. Then, a fly-eye lens 103 is disposed near the rear focal plane of the collector lens 102.
The fly-eye lens 103 forms images of the light source 101 (light source image) near the exit plane of the respective lens elements thereof.
Note that, in this drawing, the light source image is schematically depicted in the form of a filament (which is the same in the other drawings). FIG. 9 shows a case where the light source image is relayed to near the front focal plane of the condenser lens 105 through a relay optical system 104.
In this case, light fluxes L1 and L2 which are passed through the lens elements disposed at the both ends of the fly-eye lens 103 are, after respectively forming the light source images on the exit side of the fly-eye lens 103 and in the vicinity of the front focal plane of the condenser lens 105, collimated and are incident on the same position on the sample surface O.
Incidentally, a microscope is generally constituted as being capable of various kinds of observation, so that an objective lens of the microscope is frequently replaced one having a different magnification if needed by the user.
Objective lenses having magnifications different from each other are, as having different properties from each other, different in terms of an illumination state for exhibiting the properties thereof.
Specifically, an objective lens having a high magnification (high magnification objective lens) demands of the microscopic illumination apparatus 100 an illumination having a small field of view and a high numerical aperture NA (for illuminating a small area with a light flux having a large maximum angle of incidence).
On the other hand, an objective lens having a low magnification (low magnification objective lens) demands of the microscopic illumination apparatus 100 an illumination having a large field of view and a low numerical aperture NA (for illuminating a large area with a light flux having a small maximum angle of incidence).
In addition, the condenser lens 105 is seldom replaced and the same condenser lens is used for various types of objective lenses of a comparatively wide magnification range from 4 times to 100 times.
Accordingly, in the microscopic illumination apparatus 100, an aperture stop 107 of the condenser lens 105 and a field stop 106 both of which are provided inside the apparatus are appropriately adjusted.
As seen from FIG. 9, the field stop 106 is adapted to restrict each of the light fluxes emitted from a plurality of light source images, thereby restricting the diameter of each of the light fluxes entering the sample surface O.
The aperture stop 107 is adapted to restrict the light fluxes emitted from a part of the plurality of light source images, thereby restricting the light fluxes having a large angle of incidence, out of the light fluxes entering the sample surface O.
In FIG. 9, a view (a) schematically shows light source images when the field stop 106 is open and the aperture stop 107 is stopped down, while a view (b) such light source images when the field stop 106 is stopped down and the aperture stop 107 is open, respectively.
In this case, in the conventional microscopic illumination apparatus 100 shown in FIG. 9, when an illumination state is set for the high magnification lens, the diameter of the aperture stop 107 is increased and the diameter of the field stop 106 is reduced. On the other hand, when the illumination state is set for the low magnification lens, the diameter of the aperture stop 107 is reduced and the diameter of the field stop 106 is increased.
More specifically, when the illumination state is set for the low magnification lens, the outer light source images out of the plurality of light source images are restricted by the aperture stop 107, as shown in (a) at the lower right of FIG. 9. When the illumination state is set for the high magnification lens, the brightness of the plurality of light source images is respectively restricted by the field stop 107, as shown in (b) at the lower right of FIG. 9 (in the views at the lower right of FIG. 9, the bright images are indicated by the solid lines, while the dark images are by the dotted lines).
The size of a light source image formed by each lens element of the fly-eye lens 103 is determined in accordance with a ratio of the focal length of the collector lens 102 and the focal length of each lens element of the fly-eye lens 103. If the focal length of the collector lens 102 is fixed, the size of the formed light source image is small when the focal length of each lens element of the fly-eye lens 103 is shortened, while the size of the formed light source image is large when the focal length of each lens element of the fly-eye lens 103 is elongated.
Generally, the brightness of a specimen at observation is determined in accordance with an areal ratio of the size of the light source image occupying the area of the pupil of the objective lens. Accordingly, a specimen can be observed with brighter illumination when a light source image formed by each lens element of the fly-eye lens is larger.
However, in the conventional illumination apparatus, as described above, the condenser lens is not replaceable. As a result, a range which can be illuminated becomes smaller when the light source image is larger.
Accordingly, in case of observation at a low magnification, it is necessary to limit the size of the light source image, in order to secure a sufficient size of the field of view. Thus, there arises a problem that the brightness of the illumination can not be increased more.